This invention relates to a tapered roller bearing for use in rotating portions of automobiles, railcars, machines for manufacturing steel, machine tools, and particularly construction machines.
FIG. 13 shows tapered roller bearings A used in a rotating portion of a hydraulic excavator as a construction machine. More specifically, the tapered roller bearings A are mounted in a travel speed reducing sprocket S of the hydraulic excavator around which a crawler C is trained. As shown in FIG. 7, such a tapered roller bearing A comprises an outer ring 1, an inner ring 2, a plurality of tapered rollers 3 mounted between the outer and inner rings 1 and 2, and an annular retainer 4 made of synthetic resin and retaining the tapered rollers 3 so as to be circumferentially spaced apart from each other. The inner ring 2 is formed with a raceway 2a and includes a small-diameter flange 2b at the small-diameter end of the raceway 2a and a large-diameter flange 2c at the large-diameter end of the raceway 2a. Such a tapered roller bearing is disclosed in JP patent publication JP 2003-287033A.
As shown in FIG. 10, the annular retainer 4 comprises two axially spaced apart annular portions 4a, and a plurality of circumferentially equidistantly spaced apart crossbars 4b that extend between the two annular portions 4a. Pockets 5 are defined by the annular portions 4a and the adjacent crossbars 4b. Each tapered roller 3 is received in one of the pockets 5 as shown in FIG. 8A. With the tapered rollers 3 received in the respective pockets 5, the inner ring 2 is inserted into the retainer 4 so that the tapered rollers 3 are supported by the inner ring 2 (as shown in FIGS. 8B to 8D and FIGS. 9A to 9C). The thus assembled inner ring 2 and the retainer 4 are inserted in the outer ring 1 to assemble the tapered roller bearing A (FIG. 7).
Such a tapered roller bearing A can be used in rotating portions of ordinary industrial machines such as automobiles, railcars, machines for manufacturing steel, machine tools and construction machines. Among such bearings A, a bearing used in a construction machine has a large diameter, so that it is difficult to ensure enough rigidity of a retainer 4, which is made of synthetic resin, for such a bearing. One possible way to increase the rigidity of the retainer 4 is to increase the thickness of the retainer 4 (i.e. the radial thickness of the crossbars 4b of the retainer).
As shown in FIGS. 12A and 12B, the retainer 4 of such a tapered roller bearing A retains the tapered rollers 3 such that the tapered rollers 3 contact the crossbars 4b of the retainer 4 along lines b disposed radially outwardly of the pitch cone p of the tapered rollers 3 (conical enveloping surface defined by the axes of the tapered rollers 3) to prevent separation of any of the tapered rollers 3 from the inner ring 2 with the tapered rollers 3 held in position by the retainer 4. Thus, during operation of the tapered roller bearing A, radially outward loads are applied to the retainer 4 from the tapered rollers 3. If the thickness of the retainer is simply increased radially outwardly to increase its rigidity, the retainer 4 may interfere with the raceway of the outer ring 1. In the arrangement of FIGS. 12A and 12B, the surface 8b defined by the radially inner surfaces of the crossbars 8b is disposed radially outwardly of the pitch cone p.
Also in the arrangement of FIGS. 12A and 12B, in order to prevent separation of the tapered rollers 3 with the tapered rollers 3 supported on the inner ring 2, the maximum outer diameter D2 of the small-diameter flange 2b of the inner ring 2 is determined to be larger than the diameter d3 of the inscribed circle inscribed in the small-diameter end surfaces of the tapered rollers 3 with the tapered rollers 3 held by the retainer as shown in FIG. 7. (The diameter d3 is equal to the minimum diameter of the conical enveloping surface defined by tangents of the inner ring raceway 2a and the tapered rollers 3, i.e. the diameter of the inner ring raceway 2a at its small-diameter end).
Thus, when the inner ring 2 is inserted into the retainer 4 with the tapered rollers 3 held by the retainer 4, the tapered rollers 3 are pushed radially outwardly by the small-diameter flange 2b before being set on the raceway 2a of the inner ring 2.
If the retainer 4 is made of iron (see JP patent publication 2003-287033A; FIG. 6), the crossbars (7 in the figures of publication 2003-287033A) of the iron retainer 4 are plastically deformed arcuately so that the diameter d3 of the inscribed circle becomes larger than the maximum outer diameter of the small-diameter flange 2b. After the tapered rollers 3 are completely received in the inner ring 2, the crossbars are again plastically deformed until they become straight to prevent separation of the tapered rollers 3.
If the retainer 4 is made of synthetic resin, since such a retainer 4 is an integral member, the crossbars are elastically deformed when the inner ring 2 is inserted into the retainer 4 because the retainer is pushed by the small-diameter flange 2b through the tapered rollers 3.
The small-diameter flange 2b of the conventional inner ring 2 shown in FIG. 7 has an outer peripheral surface 6 that is parallel to the axis c of the bearing A or only slightly inclined (by an angle γ) radially outwardly toward the large-diameter portion 2c with respect to axis c. On the other hand, the angle α2 of the raceway 2a of the inner ring 2 with respect to the axis c of the bearing A (central angle of the inner ring 2) is usually substantially larger than the angle γ (α2>γ).
Thus, when the inner ring 2 is inserted into the retainer 4, the retainer 4 is first radially outwardly pushed by the small-diameter flange 2b through the tapered rollers 3. When the inner ring 2 is further inserted into the retainer 4, the edges of the large-diameter end surfaces of the tapered rollers 3 contact the raceway 2a because in this state, the tapered rollers 3 are supported on the outer periphery 6 of the small-diameter flange 2b and the angle α2 of the raceway 2a is greater than the inclination angle γ of the outer periphery 6 of the small-diameter flange 2b. Thus, the retainer 4 is further pushed radially outwardly when the tapered rollers 3 contact the raceway 2a. In other words, the retainer 4 is pushed radially outwardly by the inner ring 2 through the tapered rollers 3 in two stages, i.e. first by the outer periphery 6 of the small-diameter flange 2b and then by the raceway 2a. This tends to prematurely deteriorate the retainer 4 of synthetic resin.
Further, in the conventional arrangement of FIG. 7, the maximum outer diameter D2 of the small-diameter flange 2b is far greater than the diameter d3 of the circle inscribed in the small-diameter end surfaces of the tapered rollers 3 with the tapered rollers 3 held by the retainer 4. Thus, when the inner ring 2 is inserted into the retainer 4, the small-diameter annular portion 4a of the retainer 4 is significantly deflected radially outwardly. This may result in cracks X (FIG. 10) at joint portions between the small-diameter annular portion 4a and the crossbars 4b or in the worst case, breakage of the retainer 4.
In order to reduce the manufacturing cost, as shown in FIGS. 11A and 11B, the retainer 4 of synthetic resin is ordinarily formed by injection molding using two molds B1 and B2 that are separable from each other in the axial direction of the bearing A. The mold B1 has portions for forming the pockets 5 of the retainer and a cavity for forming the large-diameter annular portion 4a of the retainer 4. The mold B2 has portions for forming cutouts 5b (see FIG. 10) in both sides 5c of each crossbar 4b so that the molds B1 and B2 are axially separable from each other (movable in the directions opposite to the directions shown by the arrows in FIG. 11A) after forming the retainer 4. That is, it is essential to form the cutouts 5 in order for the molds B1 and B2 to be axially separable from each other after forming the retainer. Also, to permit axial separation of the molds B1 and B2, the side surfaces 5c of the crossbars 4b are formed into flat (FIG. 12A) or arcuate (FIG. 12B) tapered surfaces.
Because the cutouts 5b are formed on the side surfaces 5c of the crossbars 4b and the side surfaces 5c of the crossbars 4b are flat or arcuate tapered surfaces, the tapered rollers 3 received in the pockets tend to be brought into line contact with the side surfaces 5c not over the entire axial length thereof but only over part of the axial length thereof, and in the worst case, the tapered rollers 3 may be brought into point contact with the side surfaces 5c. 
If the length of line contact therebetween is short or if they are brought into point contact with each other, the tapered rollers 3 cannot stably rotate about the axis of the bearing. Also, the retaining force tends to concentrate on the small-diameter end of the retainer 4, which may result in cracks at joint portions X between the small-diameter annular portion 4a and crossbars 4b or breakage of the retainer 4.